Typically, tetrazoles are prepared from the reaction of an hydrazoic acid source (e.g. sodium azide and ammonium chloride) and an acceptor group such as a cyanate or a nitrile (Organic Reactions, 3, 307 (1946)). For example, heating an aryl or alkyl nitrile with hydrazoic acid, generated from sodium azide and hydrochloric acid, in an inert solvent in a sealed tube for 96-120 hours at temperatures from 120.degree.-150.degree. C. affords the 5-aryl or 5-alkyltetrazole (Mihina, J. S., Herbst, R. M. J. Org. Chem. 15, 1082 (1950)). Alternatively, 5-aryl tetrazoles can be prepared by brominating the appropriately substituted tolyl compound, converting the resulting dibromo-compound to the diazido-compound with sodium azide and then cyclizing by heating in refluxing dimethylformamide to give the tetrazole (Jpn. Kokai Tokkyo Koho JP 59 98,023).
Hydrazoic acid itself is extremely poisonous, explosive, cannot readily be used in stainless steel reactors and has a low boiling point (37.degree. C.). An alternative source of azide anion is trialkyltin azide (European Patent Application Number 291969). Trialkyltin azide is prepared from a trialkyltin chloride, which is volatile and toxic, and sodium azide. This reagent must be used in stoichiometric amount. Trimethylsilyl azide has also been cited as an azide source reported to react with benzonitrile (Ettenhuber, E., Ruhlmann, K. Chem. Ber. 101,743 (1968)).
Most of the above-mentioned processes involve the use of hydrazoic acid or an azide source which forms hydrazoic acid in situ and, generally, elevated temperatures are required for reaction. These reaction conditions can be hazardous. Therefore, there is a continuing need for improved processes for the preparation of tetrazoles.